Titanium alloys are valued for their strength, light weight, and durability in industrial and consumer applications. Unlike metals such as silver or copper, titanium alloys are resistant to traditional tarnishing and corrosion, making them a low-maintenance choice for items ranging from aerospace components to jewelry. This resistance results from an instantaneous chemical reaction that creates a stable surface layer.
Why Titanium Alloy Resists Tarnish and Corrosion
Titanium’s resistance to chemical degradation is due to a natural process called passivation. When titanium metal is exposed to air or moisture, it instantly reacts with oxygen to form a thin, protective surface film composed primarily of titanium dioxide. This oxide layer is stable, continuous, and highly adherent to the underlying metal, acting as a powerful barrier against further reaction.
The titanium dioxide film is chemically inert, preventing corrosive agents from reaching the metal underneath. In its natural state, this protective film is extremely thin, measuring only a few nanometers, which makes it transparent. If the surface is scratched, titanium’s high affinity for oxygen allows the protective oxide layer to spontaneously re-form, or “heal,” in any environment containing oxygen or water. This self-healing mechanism is the primary reason titanium does not tarnish.
Color Shifts Caused by Heat and Intentional Processes
While titanium does not tarnish through chemical degradation, its color can change if the thickness of the protective oxide layer is manipulated. This color change is not corrosion but a structural effect resulting from light interference. When the oxide layer thickens, light waves reflecting off the oxide surface and the underlying metal interact. This interaction, known as thin-film interference, causes certain wavelengths of white light to be amplified or canceled out, resulting in a perceived color.
Anodization
One intentional method for changing titanium’s color is anodization, an electrochemical process that precisely controls the growth of the oxide film. The titanium piece is submerged in an electrolyte bath and connected to a power source; the applied voltage determines the final thickness of the oxide layer.
Increasing the voltage from approximately 15 to 110 volts increases the oxide thickness, shifting the visible color through the spectrum, progressing from bronze and blue to purple, gold, and green. The resulting colors are not dyes or pigments but are a permanent part of the metal’s protective surface, maintaining the material’s structural integrity and corrosion resistance.
Heat Tinting
Color shifts can also occur through high heat exposure, a process known as heat tinting or thermal oxidation. When titanium is heated in an oxygen-rich atmosphere, the metal reacts with the air, causing the natural oxide layer to thicken.
The temperature reached dictates the final color: lower temperatures produce straw or gold hues, while higher temperatures lead to blues, purples, and sometimes dark gray or black. For example, 500°C may produce a blue color, while 400°C might result in a purple shade. This phenomenon is often seen on titanium exhaust components or engine parts that reach high operating temperatures.
Practical Care for Maintaining Surface Luster
Maintenance focuses on preserving titanium’s physical appearance by cleaning away surface residues. Everyday use causes dirt, oils, and skin residues to accumulate, which can make the metal appear dull or discolored. Cleaning involves using a solution of warm water and a mild, detergent-free dish soap.
Soak the item briefly to loosen grime, then gently scrub it with a soft cloth or soft-bristled brush. Rinse the piece thoroughly with clean water and immediately pat it dry with a soft, lint-free cloth to prevent water spots.
For anodized or colored titanium, use extra care by avoiding harsh chemicals and excessive scrubbing, as vigorous rubbing can damage the thin, color-producing oxide layer. While titanium is durable, deep scratches on a polished finish may require professional polishing to restore the original luster.